The present invention relates to underground geologic and fossil fuel investigations and, more particularly, to a method and apparatus for determining the electrical resistivity of a formation in a borehole.
Electrical resistivity of formations surrounding boreholes is important in geological studies and investigations for fossil fuels. Because differing materials which might make up a formation have different resistivities, a measurement of the resistivity provides an indication of the formation make-up.
There now are basically two different approaches for measuring resistivity from boreholes. One is to position a so-called "electric" log instrument in the borehole. Such instrument forces current from the borehole through the external formation and measures the effect on such current of the make-up of the formation, by detecting resulting voltages at various locations. This type of log has been found to be particularly effective for obtaining resistivity measurements in high-resistivity formations. It often, though, requires a conductive borehole fluid to operate. This approach has also been adapted to measure-while-drilling (MWD) tools by assembling measurement electrodes on the metal drill collar normally provided between the actual drill and the main part of the drill stem. Because such metal drill collars are electrically conductive, the measurement electrodes must be insulated from one another to prevent shorting. The insulation provided on the drill collar in the past for this purpose has been exposed and consequently subject to severe erosion during actual drilling. Instruments and other arrangements utilizing the forced current approach are described in U.S. Pat. Nos. 2,963,640 and 4,451,789.
The second basic approach that has been used has been an induction approach. One or more (typically an array) of solenoids or the like are positioned in the borehole to develop a primary alternating magnetic field in the formation. This will induce a circulating electrical current within the formation, and the resulting secondary magnetic field is measured. This approach is generally effective in low-resistivity formations of the type likely to be encountered in sedimentary basins which contain petroleum reservoirs. Induction log instruments also have been found to work when the borehole includes a high-resistivity fluid. The approach does not lend itself well to MWD, though, because of the necessity of providing deep circumferential cuts or grooves in the outer diameter of the drill collar to avoid easily damaged protrusions. In this connection, it must be remembered that the ability of an antenna to generate a primary magnetic field or to sense an induced secondary field, depends on the amount of non-conducting cross-sectional area that such antenna has. The resulting deep undercuts in the outer diameter of the metal drill collar weakens it unacceptably. U.S. Pat. Nos. 2,919,396; 3,383,586; and 4,609,873 disclose instruments utilizing induction.
Both of these approaches rely on the formation of a primary electromagnetic field which is not distorted because of any electromagnetic conductivities other than those which might be caused by the formation to be measured. Instruments and other constructions utilizing either of these approaches normally operate at frequencies in the range of 10 to 200 kHz. The frequency of operation is high enough to avoid low-frequency noise but low enough to neglect capacitive current components, simplifying the equations that must be solved to obtain a description of the electromagnetic fields in the formation. A version of an induction instrument known as an "electromagnetic wave resistivity" (EWR) instrument has recently come into use. This name derives from its operation at a high enough frequency (about 2 MHz) that capacitive current cannot be neglected, and the equations describing the surrounding fields and the propagating wave modes must be considered. Although the geometry of EWR instruments is generally similar to that of conventional induction instruments, the shallower undercut permitted by its high-frequency operation has resulted in its use with standard MWD collars. However, the attenuation of waves travelling in a conducting formation increases with frequency. The result has been EWR arrangements provide lower depth of investigation in low resistivities than typically are of interest. As an example of the attenuation, at a resistivity of 0.2 ohm-meters the attenuation is greater than one decibel per inch of travel. In this connection, it also must be remembered that during MWD measurement it is necessary that the primary electromagnetic field which is induced travel through fluid (typically called a drilling "mud" ) which is circulated through the borehole.